1. Field of the Invention
The present invention relates to an imaging element for making an off-axis beam with a field angle become incident on a diffraction optical element and, more particularly, to an imaging element suitable for a variety of optical systems such as a photographic camera, video camera, binocular, projector, telescope, microscope, and copying machine, in which beams in the wavelength range used concentrate on a specific order (design order) and a high diffraction efficiency can be obtained in the wavelength range used.
The present invention also relates to an image reading apparatus which has a controller for generating various control signals in an apparatus used for a copying machine, image scanner, facsimile apparatus, multifunctional printer and the like each of which has an imaging element having a diffraction optical element.
2. Related Background Art
An optical system has a variety of aberrations, and optical elements are so assembled as to correct these aberrations. Of all the aberrations generated in an optical system, chromatic aberration is conventionally reduced by combining glass materials having different dispersion characteristics. In the objective lens of a telescope or the like, a low-dispersion glass material and a high-dispersion glass material are used to form positive and negative lenses, respectively, and these lenses are combined to eliminate on-axis chromatic aberration. When the number of constituent lenses is limited, or usable glass materials are limited, chromatic aberration cannot be satisfactorily eliminated.
As opposed to the conventional method of reducing chromatic aberration by a combination of glass materials, methods of reducing chromatic aberration by arranging a diffraction optical element (to be referred to as a diffraction grating hereinafter) having diffraction action on a lens surface or in part of an optical system are disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-Open Patent Application Nos. 4-213421 and 6-324262, and U.S. Pat. No. 5,044,706. Such a method uses a physical phenomenon wherein the refraction and diffraction surfaces of an optical system exhibit chromatic aberration outputs in opposite directions with respect to a light ray having a given reference wavelength.
This will be briefly described with reference to FIG. 9. A diffraction optical element 11 is placed in air with a refractive index of 1 and is perpendicular to an optical axis 13. Diffracted light emerges in a diffraction direction xcex8 of a light ray A parallel to the optical axis 13:
P sin xcex8=mxcexxe2x80x83xe2x80x83(1)
where P is the periodic pitch of the diffraction grating 12, m is the order of diffracted light, and xcex is the wavelength.
FIG. 9 shows the periodic structure in only one direction. When such a periodic structure is built rotationally symmetrically about the optical axis or the like, and the periodic pitch of the diffraction grating is gradually changed, the resultant annular structure having this periodic structure serves as a lens. A lens using such diffraction action has a larger diffraction angle at a longer wavelength with a given order according to equation (1). With this lens, the positional relationship between the imaging points depending on wavelengths is opposite to that of a refractive lens having power in the same direction. The above references mainly use this principle to correct aberration (chromatic aberration).
In refraction, one light ray is one light ray upon refraction. In diffraction, however, one light ray is diffracted into light components of the respective orders. When a diffraction optical element is used as a lens system, the diffraction grating structure is determined so that the beams in the wavelength range used concentrate on a specific order (to be referred to as a design order hereinafter). When the intensities of light beams concentrate on the specific order, the direction of remaining diffracted light is represented by equation (1), but its intensity is low. When the intensity is zero, no diffracted light is present.
To increase the diffraction efficiency of mth-order diffracted light, if a phase difference of 2 xcfx80m is imparted to the optical-path light rays in the diffraction direction, the light rays are brought to interference and strengthened.
To impart a phase difference of 2 xcfx80m to mth-order diffracted light in a transmission diffraction grating, the following condition must be satisfied:
2 xcfx80m=2 xcfx80d(nxe2x88x921)/xcexxe2x80x83xe2x80x83(2)
where d is the height of the grating and n is the refractive index of the material of the grating. When condition (2) holds between the respective pitches, the diffraction efficiency is maximized.
The detailed structure of a diffraction optical element for obtaining this diffraction action is called a kinoform. Known examples of the kinoform are a kinoform with a continuous portion for imparting a phase difference of 2 xcfx80, a kinoform having a binary shape approximating a continuous phase difference profile stepwise, and a kinoform obtained by approximating a fine periodic structure into a triangular shape. Such a structure is formed on the surface of a flat plate or the surface of a lens to exhibit the diffraction effect. Such a diffraction optical element is manufactured by cutting or a semiconductor process such as lithography.
Such a diffraction optical element is excellent in effect for particularly correcting chromatic aberration occurring on a refraction surface upon glass material dispersion. The period of the periodic structure is changed to exhibit the effect of an aspherical lens. The periodic structure can greatly reduce the aberration.
The known examples reduce various aberrations and particularly chromatic aberration due to the diffraction effect. The effect of incorporating a diffraction optical element in an optical system can be confirmed on an aberration chart. When the diffraction efficiency of diffracted light contributing to a reduction in aberrations is not high, this diffracted light is not present in practice. The diffraction efficiency of a light ray for reducing aberration must therefore be sufficiently high. When light rays with an order different from the design order are present, these light rays form an image at a position different from that formed by the light ray of the design order to cause flare or ghost, thereby reducing the image contrast. The diffraction efficiency profile and the behavior of light rays with orders different from the design order must also be carefully considered.
FIG. 10 shows the spectral transmission characteristic of a general optical system. In FIG. 10, the wavelength is plotted along the abscissa and the spectral transmittance is plotted along the ordinate. This spectral transmission characteristic is determined by light absorption and reflection at a refraction surface of glass. A spectral transmission characteristic matching an evaluation target in the wavelength used is required for this optical system.
FIG. 11 shows a diffraction efficiency characteristic with respect to a specific diffraction order when a diffraction optical element is formed on a given surface. In FIG. 11, the wavelength is plotted along the abscissa and the diffraction efficiency is plotted along the ordinate. This diffraction optical element is designed so that the diffraction efficiency maximizes in the first order (indicated by a solid line in FIG. 11) in the wavelength range used. That is, the design order is the first order. The diffraction efficiencies of the diffraction orders (first orderxc2x1first order) adjacent to the design order are also shown in FIG. 11. As shown in FIG. 11, the diffraction efficiency in the design order maximizes at a given wavelength (to be referred to as a design wavelength hereinafter) and gradually decreases at remaining wavelengths due to the following reason. Although the thickness of the grating which makes the phase difference 2xcfx80 is exhibited in equation (2), when the thickness of the grating is so set as to satisfy this condition at the design wavelength, the condition does not hold slightly at other wavelengths to result in a decrease in diffraction efficiency.
For example, as shown in FIG. 12, assume that the microstructure of the diffraction grating 12 forming the diffraction optical element 11 is formed by a stepwise binary structure. In this case, when the design wavelength for the first-order light of the diffraction optical element 11 is 530 nm, an actual grating structure is formed such that each step has a thickness of 143.7 nm obtained by dividing by eight a thickness d=1150 nm obtained when m=1, xcex=530 nm, and n=1.461 for 2 xcfx80m=2 xcfx80d (nxe2x88x921)/xcex. In this case, the diffraction efficiency at the design wavelength is about 95%, the diffraction efficiency for the first-order light at the wavelength of 400 nm is about 67%, and the diffraction efficiency for the first-order light at the wavelength of 650 nm is 85%. Care must be taken in an optical system using the diffraction effect such that the design wavelength of this system must be set near the center of the wavelength range used. When only the diffraction efficiency in the design order is taken into consideration, it must be considered in the same manner as in the spectral transmission characteristic.
The spectral transmission characteristic of the entire optical system in the design order is defined as:
xcex7(xcex)=xcex7LENS(xcex)xc3x97xcex7DOE(xcex)
where xcex7LENS is the spectral transmission characteristic except the diffraction surface of the optical system including the diffraction optical element as a function of the wavelength, and xcex7DOE is the diffraction efficiency of the diffraction optical element. When a diffraction surface having the diffraction efficiency shown in FIG. 11 is added to an optical system having the spectral characteristic shown in FIG. 10, the spectral transmission characteristic in the design order is as shown in FIG. 13. In the wavelength range used, the diffraction efficiency in the design order is preferably high.
The influence of diffracted light of orders other than the design order will be described below. The light of orders other than the design order appears on an evaluation surface in a defocused state. This will briefly be explained. Assume that the design order is the first order, and that the power of a lens having a diffraction effect is positive. In this case, diffracted light beams of orders (second, third, . . . ) higher than the design order have larger diffraction angles according to equation (1) and form images at a position before the primary imaging position. The diffraction position value increases with an increase in order from the design order. Similarly, the diffracted light beams of orders (zero order, minus first order, . . . ) lower than the design order form images at positions behind the primary imaging position. Since the evaluation surface is located at the diffracted light imaging position of the design order, the diffracted light beams of orders other than the design order appear on the imaging surface in defocused states.
Since all the diffracted light beams of orders much larger or smaller than the design order are greatly blurred on the evaluation surface, they do not contribute to imaging and are added on the entire surface in the state of flare.
On the other hand, the diffracted light beams of orders (first orderxc2x1first order) adjacent to the design order do not resolve in the spatial frequency range for evaluating the imaging performance. These light beams form images which are not perfectly blurred but resolve in a low spatial frequency range. For this reason, when the diffraction efficiency of such a diffraction order is high, a spot with a considerably large side lobe around the diffracted light of the design order is formed, thus degrading the optical performance. The diffraction efficiencies of the diffraction orders (zero order and second order) near the design order are almost zero, as shown in FIG. 11. The diffraction orders away from the design wavelength have several % diffraction efficiencies. A light amount integrated in the wavelength range used is about 2%, and a light amount of as small as about 0.5% is obtained depending on the type of photosensitive object placed on the evaluation surface. An image corresponding to this light amount is blurred on the evaluation surface, and the light amount per unit area is very small. Generally, the light cannot be detected as a side lobe.
However, when an optical system using this diffraction effect is used for a lens (phototaking system) or the like in a camera, a special condition must be considered. In a camera, a film or CCD is used as the evaluation surface, and phototaking conditions (object and exposure) vary. If a high-luminance light source is present in part of an object, the high-luminance light source is saturated exceeding the optimal exposure of the film or CCD, and the remaining object portion is adjusted to the optimal exposure. Under these conditions, phototaking is done. In this case, since the light source has an exposure value several times the optical exposure, the diffracted light beams of diffraction orders adjacent to the design order are also multiplied several times. A side lobe may be observed as backlight around the light source.
When the diffraction optical element is used for an optical system having a variety of exposure conditions like a camera, the efficiencies of the diffraction orders adjacent to the design order must preferably be reduced.
As shown in FIGS. 14, 15, and 16, a diffraction optical element is formed from at least two diffraction gratings stacked on each other and made of materials having different Abbe""s numbers (dispersions). With this arrangement, the diffraction efficiency can be enhanced over the entire wavelength region used while the diffraction efficiencies of orders adjacent to the design order can be reduced. When the diffraction optical element is used for an optical system, various aberrations such as chromatic aberration or the like can be properly corrected. The diffraction optical element is suitable for a variety of optical systems such as a phototaking camera, video camera, binocular, projector, telescope, microscope, and copying machine. Various diffraction optical elements and optical systems using them are disclosed in Japanese Laid-Open Patent Application Nos. 10-133149 and 11-223717.
The conventional diffraction optical element is designed such that the diffraction efficiency of the design order (first order) is high in the wavelength range used, and diffraction efficiencies of orders adjacent to the design order are reduced when a light beam (light ray) is perpendicularly incident on the conventional diffraction optical element. However, no consideration is made for a beam with a field angle, i.e., the field angle characteristics of diffraction efficiency.
For example, as shown in FIG. 17, assume that the pitch of a diffraction grating is 100 xcexcm, that the material of a diffraction grating 91 is UV1000 (nd=1.6363, xcexdd=23.0) available from Mitsubishi Chemical Corp., that the material of a diffraction grating 92 is RC8922 (nd=1.5129, xcexdd=51.0), and that the grating depths of the gratings 91 and 92 are set to d1=7.432 xcexcm and d2=10.295 xcexcm, respectively. In this case, when the incident angles are set to xcex8=0xc2x0, xc2x15xc2x0, and xc2x110xc2x0, the diffraction efficiencies are shown in FIG. 18. As can be apparent from FIG. 18, when the field angle increases, the diffraction efficiencies of orders adjacent to the design order increase in the visible range. Particularly, the incident angle on the minus (xe2x88x92) side poses a problem because the diffraction efficiency of secondary light increases at a wavelength near 500 nm to which the human eye is sensitive.
In an optical system in which light beams at various angles are present, preferably, the diffraction efficiency of the design order (first-order diffracted light) is high in the wavelength range used and the diffraction gratings of orders adjacent to the design order are reduced even if the beams are incident on the grating surface at various angles.
It is an object of the present invention to provide an imaging lens using the above diffraction optical element, wherein of beams having field angles and incident on the diffraction optical element, beams having incident angles larger than the incident angle of a principal light ray of the beams incident on the grating surface are partially shielded by a light-shielding member, so that the diffraction efficiencies of orders adjacent to a design order are kept low and hence imaging performance almost free from flare can be achieved even if a beam with a field angle is incident.
In one aspect of the invention, there is provided an imaging element including a diffraction optical element on which an off-axis beam with a field angle is incident and a stop, said imaging element comprising a limiting member located at a position farther away from said diffraction optical element than said stop, wherein of off-axis beams having field angles and incident on said diffraction optical element, a beam having an incident angle larger than an incident angle of a principal ray of the beams incident on a grating surface of said diffraction optical element is partially shielded by said limiting member, wherein the incident angle is an angle defined by the beam and the normal to the grating surface.
In a further aspect of the imaging element, said diffraction optical element is disposed to face said stop.
In a further aspect of the imaging element, said limiting member is located outside a lens constituting said imaging element along an optical axis with respect to said diffraction optical element as the center.
In a further aspect of the imaging element, a grating pitch of said diffraction optical element is reduced from an on-axis side to an off-axis side.
In a further aspect of the imaging element, said imaging element comprises a diffraction optical element in which first and second diffraction gratings with different Abbe""s numbers and different refractive indices are arranged on a substrate such that grating surfaces thereof face each other.
In a further aspect of the imaging element, said limiting member is disposed on an incident and/or exit side of said imaging element.
In a further aspect of the imaging element, one of said first and second diffraction gratings, which is made of a material having a higher refractive index is disposed on the original surface side.
In a further aspect of the imaging element, said one diffraction grating made of the material having a higher refractive index has an incident surface as a flat surface.
In a further aspect of the imaging element, each of said first and second diffraction gratings is formed on a flat substrate.
In a further aspect of the imaging element, said first and second diffraction gratings are so stacked as to correspond to each other for each pitch.
In a further aspect of the imaging element, said limiting member comprises a second stop.
In a further aspect of the imaging element, said limiting member comprises a reflecting mirror.
In a further aspect of the imaging element, said limiting member is provided on a surface of a lens constituting said imaging element.
In a further aspect of the imaging element, said imaging element comprises a lens for forming on a reading element surface an image of image information formed on an original illuminated with a beam from a light source.
In a further aspect of the imaging element, said imaging element comprises a positive first lens, a negative second lens, a negative third lens, and a positive fourth lens in the order named from the original surface side, said stop is disposed between said negative second lens and said negative third lens, and said diffraction optical element is disposed near said stop.
In a further aspect of the imaging element, letting D be a diameter of said stop, f be a focal length of said imaging element, Y be a maximum object height, and xcex2 be an absolute value of an imaging magnification, a distance L from said stop to said second stop serving as the exit-side light-shielding member along the optical axis falls within a range:
(0.4xc3x97Dxc3x97f)/(Yxc3x97xcex2) less than L less than (1.2xc3x97Dxc3x97f)/(Yxc3x97xcex2).
In a further aspect of the imaging element, letting D be a diameter of said stop, f be a focal length of said imaging element, Y be a maximum object height, and xcex2 be an absolute value of an imaging magnification, a distance L from said stop to said second stop serving as the incident-side light-sheilding member along the optical axis falls within a range:
(0.4xc3x97Dxc3x97f)/(Yxc3x97xcex2) less than L less than (1.2xc3x97Dxc3x97f)/(Yxc3x97xcex2).
In another aspect of the invention, there is provided an image reading apparatus which forms on the reading element surface the image information formed on the original using said imaging element of any one of the foregoing aspects and includes a controller.